Serena Williams says the case of Chinese tennis player Peng Shuai "must be investigated" and urged people "not to stay silent" about her welfare.
Peng, 35, has not been heard from since she made sexual assault allegations against a leading Chinese government official two weeks ago.
Chinese state media has released an email attributed to Peng, but doubt has been cast about its authenticity.
"I hope she is safe and found as soon as possible," said American Williams.
Steve Simon, chairman of the Women's Tennis Association, has said the organisation is prepared to pull its tournaments out of China if it is not satisfied with the response to Peng's allegation.